1. Field of the Invention
The present invention relates to a speaker grille for covering the surface of an audio speaker installed in an automobile compartment or any other types of speaker. The speaker grille is integrally formed of a synthetic resin to include a grille plate having a plurality of through holes and a frame for supporting the outer periphery of the grille plate. This invention also relates to a molding method for the speaker grille. More particularly, the present invention relates to a speaker grille which can eliminate the generation of appearance defects such as flow mark, weld, and short in molding to thereby obtain a beautiful appearance. It must be molded by using a metal mold advantageous in strength, and also relates to a molding method for the speaker grille.
2. Description of the Related Art
In general, a speaker grille for covering the surface of an audio speaker installed in an automobile compartment or any other types of speaker is constructed of a frame formed of a synthetic resin and punching metal or a metal lattice plate fixed to the frame. Such a speaker grille has disadvantages in parts cost and assembly (mounting) cost because it is composed of a plurality of parts.
FIG. 8 shows another type of known speaker grille integrally formed of a synthetic resin to include a grille plate 1 having a plurality of through holes 2 and a frame 3 for supporting the outer periphery of the grille plate 1 (such a speaker grille will be hereinafter referred to as an integral speaker grille). In obtaining a relatively large speaker grille of this type as shown in FIG. 9, it is also known that a lattice-shaped reinforcing rib 4 is integrally formed on the back surface of the grille plate 1 to obtain sufficient strength.
Further, as shown in FIG. 10, it is also known that the through holes 2 such as punched holes or latticed holes are formed also at the boundary between the grille plate 1 and the frame 3 to make the design of the integral speaker grille similar to that of the speaker grille having a punching metal or a metal lattice plate fixed to the frame.
Such an integral speaker grille is molded by combining an upper mold (a half mold for molding the front surface of the speaker grille) having in its cavity a plurality of pins for forming the through holes 2 and a lower mold (another half mold for molding the back surface of the speaker grille) having in its cavity a groove for forming the reinforcing rib 4 as required, and then injecting a molding resin usually from the lower mold side into the metal mold composed of the upper mold and the lower mold, to avoid that a gate mark (resin injection hole mark) may be left on the front surface of the speaker grille. In some cases, a third metal mold for molding the frame 3 around the grille plate 1 is combined with the combined upper mold and lower mold.
Further, the present applicant has already proposed another type of integral speaker grille (Japanese Patent Application No. Hei 5-346365) as shown in FIG. 5. This integral speaker grille further includes a sound transmitting sheet 8 integrally provided between the grille plate 1 and the reinforcing rib 4. The speaker grille as shown in FIG. 5 is formed by disposing the sound transmitting sheet 8 such as a sheet of nonwoven fabric, woven fabric, or knitted fabric between the lower mold having the groove for forming the reinforcing rib 4 and the upper mold having the plural pins, and then injecting a molding resin from the lower mold side to pass the resin through the sheet to fill the cavity defined by the upper mold and the lower mold with the resin.
However, these integral speaker grilles in the related art have a problem in moldability. That is, appearance defects such as flow mark, weld, and short are prone to generate in molding the speaker grille. Specifically, since the plural pins for forming the punched holes or the latticed holes each having a small size are present in the cavity of the metal mold for molding the speaker grille, the molding resin injected from the lower mold side receives a large resistance during flowing in the cavity, causing insufficient filling of the cavity with the molding resin. As a result, short (cutout) is generated at a portion for forming the through holes of the grille plate, or weld and flow mark (marks of resin flow) are generated on the frame at many positions. Thus, the problem in appearance of the speaker grille is prone to occur.
This problem will now be described in more detail. In obtaining the speaker grille having the reinforcing rib 4 on the back surface of the grille plate 1 as shown in FIG. 9, the resin is charged from a central portion of the lower mold at several positions into the cavity of the metal mold. At this time, the resin is allowed to flow fast in the groove for forming the reinforcing rib 4 in the lower mold, which is lower in flow resistance than the upper mold having the plural pins. Accordingly, the resin flowing in the groove for forming the reinforcing rib 4 in the lower mold early reaches a frame forming portion around a grille plate forming portion, and the frame forming portion is filled with resin earlier than the grille plate forming portion. Thereafter, the grille plate forming portion in the upper mold is filled with resin. At this time, as shown in FIG. 7, the resin a lately reached the outer periphery of the grille plate 1, as shown by reference characters, flows into the frame forming portion already filled with the resin (see small arrows in FIG. 7), thus causing the generation of weld and flow mark. Furthermore, since the resin a flowing in the upper mold tends to flow from a portion where it has reached the outer periphery of the grille plate 1 into the frame forming portion, the resin a is not spread enough to a portion shown by reference character c in FIG. 7 where the flows of the resin a radially spread from resin injecting gates b are not overlapped each other. Accordingly, the portion c is not filled enough with the resin a, causing the generation of short (cutout). In the speaker grille having the sound transmitting sheet 8 between the grille plate 1 and the reinforcing rib 4 as shown in FIG. 5, the sheet 8 increases the flow resistance to cause a large difference in flow speed of the resin between the upper mold and the lower mold, which makes more remarkable the generation of weld, flow mark, and short.
Further, a similar problem sometimes occurs also in the speaker grille not having the reinforcing rib 4 on the back surface of the grille plate 1 not so frequently as in the speaker grille having the reinforcing rib 4. Specifically, the resin charged from the central portion of the lower mold into the cavity in molding the speaker grille not having the reinforcing rib 4 is radially outwardly spread. At this time, the resin is not always uniformly spread in such a manner as to be spread fast at a certain portion, for example, elliptically spread. Accordingly, the resin reaching the frame forming portion flows into the frame early forming portion having a low flow resistance, and the frame forming portion is filled with the resin. Thereafter, the grille plate forming portion is filled with the resin, resulting in the occurrence of the similar problem.
In addition to the problem in moldability of the integral speaker grille as mentioned above, there is another problem in handlability of the metal mold in obtaining the speaker grille having the through holes 2 also at the boundary between the grille plate 1 and the frame 3 as shown in FIG. 10. That is, the pins for forming the through holes 2 at the boundary between the grille plate 1 and the frame 3 are very thin so that each pin has a semicircular shape in cross section or the like. Accordingly, in removing a molding from the metal mold, the pins are prone to break.